Amyotrophic lateral sclerosis is a prevalent neurological disorder characterized by inexorable muscle weakness leading to death. The principal pathological finding in amyotrophic lateral sclerosis is loss of nerve cells in the anterior horns of the spinal cord, the motor nuclei of the brainstem, and the upper motor neurons of the cerebral cortex. Investigations aimed at preventing or limiting progression of amyotrophic lateral sclerosis have thus focused on the mechanisms by which neurons degenerate. A transgenic mouse model has been developed that possesses many of the pathological and clinical features of human familial and sporadic amyotrophic lateral sclerosis. As nitric oxide (NO) has been shown to mediate neuronal loss in other neurodegenerative conditions, several groups have investigated the role that NO may play in disease progression | in the transgenic model. The results have been conflicting likely because currently available inhibitors of nitric oxide synthase do not permit optimal control of NO generation within particular cell types and subcellular compartments. A novel potential strategy for regulating nitric oxide synthesis involves the enzyme arginase that can | regulate availability of arginine in the cytoplasm or mitochondria. In preliminary studies, we have shown that: 1) extracellular arginase blocks neuronal apoptosis and 2) arginase immunoreactivity is, upregulated in the spinal cord of ALS transgenic mice as well as humans with the sporadic and familial forms of amyotrophic lateral sclerosis. These preliminary results lead to the overall hypothesis to be tested in this proposal: about Interventions aimed at promoting arginase activities in microglia, astrocytes and/or motor neurons will limit availability of cell arginine for toxic NO generation and thereby diminish cell death and disease progression in amyotrophic lateral sclerosis but permit NO to, mediate its survival promoting effects in each of these cell types. We propose to test this hypothesis by: 1) determining the cell types and subcellular compartments where arginase is expressed in the normal central nervous system of humans and mice, and how the localization and levels of these isoforms change in amyotrophic lateral sclerosis as well as in a transgenic mouse! Model of amyotrophic lateral sclerosis and how this compares to the localization of NOS (all forms) in these tissues; and 2) determining whether increased arginase activity in microglia, astrocytes or neurons from control mice or mice over expressing SOD1 mutant (G93A) will abrogate NO mediated toxicity of motor neurons induced by growth factor deprivation, excitotoxins or LPS/IFN-gamma treatment. These studies promise to enhance our understanding of how arginine about metabolism, including the synthesis of NO, is regulated in the normal and abnormal nervous system.